1. Field of the Invention
The present invention generally relates to optical recording of information and more particularly to a multiple-beam optical recording system for recording information by means of a plurality of optical beams.
2. Discussion of Background
Optical scanners are used extensively for recording images in image recording apparatuses such as printers, copiers, facsimiles, scanners, and the like. Further, optical scanners are usable for various measuring purposes as well as for display devices. In order to improve the efficiency of scanning, multiple-beam optical scanners are proposed in which a plurality of optical beams are used for scanning an image formation surface simultaneously.
In a typical multiple-beam optical scanner, a plurality of optical beams each produced by an optical source such as a laser diode, are assembled with each other with minute mutual inclination angles for respective optical paths in a vertical scanning direction, such that an array of optical dots aligned in the vertical scanning direction is formed on a recording surface, after deflection and focusing of the optical beams by an optical deflector. The optical dots thus formed scan the recording surface simultaneously in a horizontal scanning direction in response to the deflection of the optical beams.
Such conventional multiple-beam optical scanners are, while capable of scanning the recording surface efficiently, have various problems associated with the use of two or more optical beams.
An example of such a multiple-beam optical scanner is known according to the Japanese Laid-open Patent Publication 7-209596 corresponding to the U.S. patent application Ser. No. 08/174,197, which describes a system in which two laser diodes disposed with a mutual separation of 25 .mu.m in the vertical scanning direction are used for the optical source. Hereinafter, the foregoing prior art will be designated as "first prior art."
In the first prior art, the optical beams produced by the laser diodes are then deflected by the same mirror surface of a rotary polygonal mirror and are exited parallel in the vertical scanning direction. Thereby, the two optical beams are in a telecentric relationship in the vertical scanning direction, and the two optical beams scan the recording surface simultaneously over a length of 302.8 mm in the horizontal scanning direction with a mutual separation of 127 .mu.m in the vertical scanning direction. As the two optical beams are telecentric in the vertical scanning direction, the "differential scan line bow" between two scanning lines formed simultaneously by the foregoing two optical beams, is suppressed within a limit of 3-5 .mu.m. The differential scan line bow will be explained in more detail in later.
Further, the Japanese Laid-open Patent Publication 5-127111, designated hereinafter as "second prior art," describes another multiple-beam optical scanner in which a rotary deflector carrying polygonal facets is disposed before a location where the multiple optical beams cross each other, for reducing the size of the polygonal mirror facets. The optical beams deflected by the rotary deflector are then focused upon a recording surface.
In such multiple-beam optical scanners applied to an image recording apparatus for recording images as a result of scanning of the optical beams over a recording surface, there occurs a problem of distortion of the recorded image on the recording surface when the optical beams are deviated in position on the recording surface in the horizontal scanning direction. Such a distortion occurs when there is an error in the shape, composition or alignment of the optical components in the image recording apparatus. Further, such a distortion occurs also as a result of change of the environmental temperature.
Thus, in order to eliminate the problem of image distortion, conventional image recording apparatuses generally use optical sensors for detecting the timing of scanning of the optical beams in the horizontal scanning direction. Such optical sensors are disposed in the vicinity of the image recording surface, and the recording of image by the optical beams is carried out in synchronization with the output of the optical sensors for avoiding the distortion of the images.
In the foregoing multiple-beam scanner of the first prior art, it should be noted that the optical beams are caused to scan over the recording surface in the horizontal scanning direction such that the optical beams, while being separated from each other in the vertical scanning direction by a distance of about 127 .mu.m, hit the respective scanning points having the same location as measured in the horizontal scanning direction. In other words, the optical beam spots formed on the recording surface by the optical beams are aligned in the vertical scanning direction in the first prior art. As the optical beams are separated only by the distance of 127 .mu.m, the multiple-beam optical scanner of the first prior art is difficult to use different optical sensors for detecting the timing of scanning of the optical beams separately.
On the other hand, there is a proposal to set the path of the optical beams to be offset from each other also in the horizontal scanning direction for facilitating the timing detection of the optical beams by using separate optical sensors as proposed in the Japanese Laid-open Patent Publication 56-67277, which will be designated as "third prior art."
According to the third prior art, a laser diode array including a plurality of laser diodes forming a monolithic optical integrated circuit is used as the optical source, wherein the laser diode array is set with a tilting such that the laser diodes in the array are aligned in an oblique direction with respect to a rotational axis of the rotary polygonal deflector. Thereby, the optical beam spots formed on the recording surface are aligned obliquely to the horizontal scanning direction, and the optical beam spots are separated from each other on the recording surface in the horizontal scanning direction by a distance of 3.75 mm. Thus, the third prior art allows the separate detection of the timing of the optical beams by using separate optical sensors.
In the multiple-beam optical scanner of the second prior art and the third prior art, in which the optical beams are offset in the horizontal scanning direction, it should be noted that the optical beams reach the scanning points on the recording surface along respective optical paths that pass different locations of the optical elements disposed between the rotary deflector and the recording surface.
In an optical image recording apparatus that uses a finely focused optical beam for scanning over an image recording surface one-dimensionally in the horizontal scanning direction, each location of an optical element forming the image recording apparatus corresponds to a scanning point on the image recording surface. In other words, a ray forming a part of the finely focused optical beam reaches a predetermined scanning point on the image recording surface after passing through respective locations of the optical elements corresponding to the scanning point. Thus, the optical elements of such optical image recording apparatuses are optimized with respect to the shape, position, and the like, such that the ray forming the horizontally scanning optical beam reaches the predetermined scanning points consecutively and exactly.
When the multiple-beam optical scanner of the foregoing second or third prior art is applied to such an image recording apparatus, in which the optical beams pass respective paths offset in the horizontal as well as vertical scanning directions, the reflection or refraction caused by the optical elements may be different between the different optical beams. Such a difference in the optical path of the horizontally scanning optical beams causes the problem of the foregoing differential scan line bow.
Hereinafter, the foregoing problem of differential scan line bow will be explained in more detail with reference to FIG. 1 showing an optical system used in an optical image recording apparatus disclosed for example in a Japanese Laid-open Patent Publication 6-123844. It should be noted that the optical system of FIG. 1 itself is designed for recording an image on a recording surface by means of a single optical beam.
Referring to the drawings, the optical system includes a laser diode 1 acting as an optical source for producing an optical beam, wherein the optical beam is caused to pass through a collimator lens 2 and a cylindrical lens 3 and hits a rotary polygonal deflector 4. The rotary polygonal deflector 4 causes a deflection of the optical beam incident thereto, and the optical beam produced as a result of the deflection reaches a recording surface 7 after reflection by a focusing mirror 5 and refraction by a cylindrical lens 8. As a result of the rotation of the polygonal deflector 4, the optical beam scans over the recording surface in the horizontal scanning direction coincident to the direction indicated by a Y-axis in FIG. 1. It should be noted that the vertical scanning direction is coincident to the X-direction on the recording surface 7.
When the multiple-beam optical scanner of the foregoing second or third prior art is applied to the optical system of FIG. 1, two optical beams C1 and C2 scan over the recording surface 7 in the horizontal scanning direction as indicated in FIG. 2, wherein it should be noted that FIG. 2 shows the optical beams C1 and C2 at respective different instances chosen such that the optical beam C2 hits the same horizontal scanning position or Y-coordinate which is previously reached by the optical beam C1. When the optical beams C1 and C2 are compared at the same instance, the horizontal scanning position of the optical beam C1 is different from the horizontal scanning position of the optical beam C2 on the recording surface 7, and the separate detection of timing of the optical beams C1 and C2 is achieved easily by using separate optical sensors.
On the other hand, the relationship of FIG. 2 clearly indicates that the angle of incidence of the optical beam C2 to the rotary polygonal deflector 4 is different from the angle of incidence of the optical beam C1 at the moment or instance when the optical beam C2 hits the same scanning point which is hit previously by the beam C1. It should be noted that there is formed a cross angle 2.DELTA..alpha. between the optical beam C1 and the optical beam C2. Thus, the optical beam C2 reaching the same scanning point on the recording surface 7 hit previously by the optical beam C1 travels along a path different from the path of the optical beam C1. This means that the optical beam C2 passes the focusing mirror 5 as well as the cylindrical lens 8 at a location different from the location which the optical beam C1 has passed. As a result of such a different in the optical path between the optical beams C1 and C2, there appears a differential scan line bow as indicated in FIG. 3.
Referring to FIG. 3 showing the scanning lines formed by the optical beams C1 and C2, it will be noted that each of the scanning lines has an undulated shape in the vertical scanning direction or Z-direction, wherein the shape for the scanning line C1 is not exactly analogous to the shape of the scanning line C2. In other words, the scanning line drawn by the optical beam C2 is not a mere translation of the scanning line drawn by the optical beam C1. This net difference between the two scanning lines.
The reason of such a differential scan line bow is attributed to the fact that the focusing mirror 5 is slightly curled about an axis parallel to the horizontal scanning direction or the Y-direction. Further, the optical beams C1 and C2 are directed so as to hit the mirror 5 at respective points that are offset with each other in the vertical scanning direction or the z-direction, in order to secure a sufficient separation in the vertical scanning direction on the recording surface 7.
FIG. 4A shows the scanning of the optical beams C1 and C2 over the mirror 5, wherein the point designated by "1" designates the beam spot formed by the beam C1 while the point designated by "2" designates the beam spot formed by the beam C2. Both of the spots "1" and "2" form respective, mutually parallel arcuate paths on the mirror 5, wherein it should be noted that the timing of the scanning is slightly different between the optical beam C1 and the optical beam C2. It should be noted that FIG. 4A shows the spot "1" and the associated spot "2" at a common instance, contrary to the representation of FIG. 2 that shows the beams C1 and C2 at different instances. In FIG. 2, the set of the spot "1" and the spot "2" moves in the Y-direction with time as the time increases from t.sub.1 to t.sub.7.
Referring to FIG. 4A, it should be noted that the separation between the beam spot "1" and the beam spot "2" in the vertical scanning direction changes with time due to the difference in the timing of the beam spots "1" and "2" on the mirror surface 5. As a result, the vertical separation between the beam spots "1" and "2" changes with the progress of scanning as indicated in FIG. 4B. Such a differential scan line bow can reach as much as 30 .mu.m, while this value of the differential scan line bow is substantial in view of the pitch of 63.5 .mu.m of the horizontal scanning lines formed by the optical beams C1 and C2.
The foregoing problem of the differential scan line bow can be eliminated by a construction proposal in the Japanese Patent Publication 64-10805 that uses use a laser diode array including laser diode elements aligned in the horizontal scanning direction, for the optical source as indicated in FIG. 5. The reference will be designated hereinafter as "fourth prior art."
Referring to FIG. 5, the optical system of the fourth prior art includes a laser diode array 51 including a plurality of laser diode elements disposed with a mutual pitch of about 100 .mu.m in the plane parallel to the plane of scanning of the optical beams. Further, the laser diode elements may be disposed with a slight mutual offset in the vertical scanning direction. The optical beams thus produced are then passed through an afocal collimating lens 52 and focused upon the recording surface 58 via a cylindrical lens 57 after deflection by a rotary polygonal deflector 56 that carries thereon mirror facets 56a.
The optical system of the fourth prior art further includes an optical system 55 between the lens 52 and the deflector 56, such that an exit pupil 54 of the lens 52 becomes conjugate to the mirror facet 56a of the rotary polygonal mirror 56 within the plane parallel to the horizontal scanning caused by the optical beams c1 and c2. The optical system 55 thereby acts as a demagnifying optical system.
It should be noted that the foregoing forth prior art merely intends to use the optical system 55 so as to reduce the diameter of the polygonal mirror that forms the optical deflector 56. On the other hand, the construction of FIG. 5 unintentionally realizes an arrangement of the optical elements such that the optical beams produced by the laser diode array 51 pass, after deflection by the deflector 56, through substantially the same optical path when the optical beams are directed to the same horizontal scanning position of the recording surface 58. Thus, the construction of FIG. 5 can successfully minimize the foregoing problem of the differential scan line bow.
On the other hand, the construction of FIG. 5 requires the optical system 55 between the afocal collimating optical system 52 and the deflector 56 as noted already, while use of such an optical system 55 requires a substantial space in the image recording apparatus. Further, use of such an optical system inevitably increases the cost of the image recording apparatus.